This invention relates to the charging of electrical energy storage components, and more particularly to a charger usable with the super capacitor energy storage elements of an aircraft electrical energy supply augmentation apparatus.
Batteries are widely used to store electrical energy. However, the use of batteries encounters numerous problems, problems largely relating to the electrochemical nature of an electrical battery, problems which include severe energy density limitations, environmental hazards, safety problems, maintenance costs, charging rate limitations, finite number of possible charge cycles and battery life, memory problems in some batteries such as the popular NiCd device, complicated charging circuits, and need for continuous replacement.
Future aircraft electrical energy supply systems in addition may involve the widespread use of a higher voltage direct current electrical energy distribution bus, a distribution involving energization by the rectified output of a polyphase alternator and voltages above the level of one hundred volts, for example. In addition to greater levels of energy availability and reduced weight of conductor metal which is possible in such distribution systems, the use of higher voltage distribution in these future aircraft is seen to offer advantages in the area of lower bus current levels, smaller bus, conductor space requirements, and better electrical transient control or filtering, particularly as to filter weight reduction. These advantages are considered to outweigh the obvious complications of conventional battery exclusion from the bus (e.g., because of the large number of individual battery cells required), absence of bus-accomplished stand alone or static engine start capability, increased hazard to personnel, and increased electrical insulation requirements. The F-22 tactical aircraft is perhaps one of the first aircraft with bus energization of this higher voltage direct current type to be considered for widespread use by the United States Air Force. Additional discussion regarding the energy bus arrangements under consideration for future aircraft and the criticality of some loads supplied from these bus arrangements is to be found in the technical paper "Power Management and Distribution for the More Electric Aircraft" by Joseph A. Weimer, presented at the Intersociety Energy Conservation Engineering Conference (IECEC) as paper No. AP-385, ASME 1995.
As an improvement to both these future high voltage aircraft systems, and to present-day lower voltage direct current bus distribution systems as well, the use of energy storage accomplished in a replacement or a supplement to the commonly encountered electrical battery, e.g., the lead-acid or nickel-cadmium battery, is envisioned. Moreover, the use of such energy storage capability disposed at several distributed locations along the physical extent of an electrical bus or disposed within particular bus load devices (such as avionics or other electronic system housings) is a part of this improved aircraft bus thinking. Such energy storage capability may take the form of one or more storage elements which can float on the bus with little energy demand or current flow during normal operating conditions, employ relatively low current and long duration recharging times and then be capable of rapidly supplying energy to accommodate brief intervals of heavy bus demand or transient loading or bus source interruption. Such arrangements could, for example, enable further reductions in bus metal mass and cross-sectional area and improve voltage regulation along the bus, i.e., could enhance the tradeoff between bus size and bus voltage regulation.
The configuration of such energy storage elements used at distributed locations along a bus for this heavy demand or transient loading improvement is now considered to reasonably include large capacitance capacitors, i.e., capacitors of the multiple farad electrical size or "super capacitor" type; such capacitors are also known as double-layer capacitors in the art. In this configuration it is notable that the capacitor is employed as an energy storage element rather than for its low alternating current impedance or other characteristics. Indeed, a super capacitor may not provide the lowest alternating current impedance available in a capacitor. Such "super capacitors" are, however, considered preferable to a battery for present energy storage uses for reasons of size, weight, reliability and decreased maintenance requirements, and are now readily available as commercial products. Capacitors of this nature are, however, most readily fabricated as units of large electrical size having moderate operating voltage capability. As noted below herein sizes such as an integral number of farads of electrical capacitance and a few tens of operating volts capability are now conveniently provided. Capacitors of this electrical rating may of course be combined in appropriate series and parallel combinations for use in the present invention.
By way of additional background it may be interesting to consider that the super capacitor element itself was first investigated by Helmholtz in 1879. According to one super capacitor arrangement, one electrode of the device is made of carbon and the other is made of a liquid electrolyte. When a voltage is applied to the carbon layer with respect to the liquid electrolyte, a thin dielectric layer is established adjacent the carbon layer particles. The effective surface area of the dielectric layer and the carbon particles is, however, extremely large. Surface areas on the order of 1000 square meters per gram of carbon material can be achieved with such electrode arrangements because of the porous surface of the carbon and the small carbon particle size. The thickness of the dielectric layer on the other hand can be extremely small--on the order of 1 nanometer. As a result, a high ratio of surface area to dielectric thickness can be obtained and surprising capacitances per unit of capacitor volume are obtainable, therefore desirable volumetric efficiency is obtained for such a capacitor. As may be surmised from a consideration of such structural details, however, questions of permissible operating voltage (i.e., the dielectric strength of the thin dielectric layer), tolerable current flow rates and resulting temperature rise, energy losses, liquid electrolyte inconvenience and physical stability of this type of super capacitor require special consideration in the capacitor's design and fabrication sequences.
The large capacitance of super capacitors nevertheless permits the storage of relatively large amounts of energy. As is well known in the electrical art, however, a change in the quantity of electrical energy stored in a super capacitor or any capacitor, unlike most battery types, involves a precisely related change in the capacitor's terminal voltage; moreover, the capacitor's stored energy quantity is a square-law function of the capacitor's terminal voltage. Therefore, in an energy storage use of such a capacitor there is a need for a power processing circuit capable of maintaining the output voltage constant while the voltage across the super capacitor decreases due to its discharge. When viewed from a different perspective this decreasing voltage relationship also dictates that the storage of useful quantities of electrical energy in a capacitor of practical electrical and physical size requires the capacitor to operate under conditions of large terminal voltage swing. This characteristic is, however, poorly suited to direct bus shunting use of such capacitors in an aircraft or in other electrical bus supplementing applications since a bus is desirably operated with very limited changes in voltage. Enormously sized capacitors are therefore required to store meaningful quantities of usable energy under the conditions of little capacitor voltage change.
The use of an electronic coupling arrangement, i.e., a device such as a direct current to direct current inverter switching circuit, is seen as an answer to this storage efficiency and other difficulties such capacitor energy storage can encounter. With such a coupling circuit providing energy transfer between a "super capacitor" storage element and the aircraft bus, the capacitor voltage can be allowed to swing through a large range and thereby provide relatively efficient (and again voltage square law-determined) energy storage while the aircraft bus voltage is held nearly constant. Such a coupling arrangement also allows a marriage of incompatible capacitor and bus voltage ratings, allows for controlled or limited current recharging of the capacitor from the bus and other advantages. With respect to the marriage of incompatible capacitor and bus voltage ratings in a bus supplement apparatus, it is perhaps helpful to appreciate that presently available super capacitors are capable of several hundred farads of capacitance within a single physical container and with an operating voltage of 3-20 volts. Such capacitors provide a stored energy density of 10-20 joules per gram of capacitor weight.
The present invention, and each of the above-identified companion inventions relating to the present invention, concern a different portion of an aircraft bus supplementing energy storage arrangement disposed generally according to this description. In the present invention a particular aspect of a capacitance charging electronic coupling between an aircraft supply bus and a "super capacitor", i.e., one aspect of a direct current charging arrangement for a super capacitor energy storage element, is considered. The present invention provides an alternative for the super capacitor charging disclosed in the above identified "Super Capacitor Battery Clone", AFD 00102, Ser. No. 08/843,408 patent application and particularly addresses one aspect of the difference encountered in use of a voltage-responsive capacitance element rather than a fixed voltage battery element for storing bus supplement energy, a difference encountered in supplying energy directly to the super capacitor storage element. Another of these patent application, the Feedforward Control of Aircraft Bus DC Boost Converter, AFD 00129, Ser. No. 08/843,428, addresses an alternate control of the super capacitor discharging operation.
The prior art is known to contain numerous arrangements for supplying energy to an electrical capacitance element. It is believed, however, that this prior art does not extend to a practical addressing of conditions encountered in supplying energy to a particular type of capacitor, the super capacitor, when this super capacitor is embodied in aircraft bus supplementation apparatus.